I've invested a fair amount of time in Rails, am happy with it. I'm not sure I'd jump for something that's "a bit better".
I've learnt to use and love Rails and it has a massive community spanning many years, so I'm not going to jump ship because others said its cool...
Thats not to say I'm ignorant to new methods of doing things, if something is better I want to try it - but why is merb better? If you compare rails 2.2 (thinking ahead) with merb, what can I do better, faster and safer which will be worth the time I put in to learn the quirks of the framework?
Merb also does not lock you in to a specific way of doing things. It's very easy to bend the framework to your will.
Rails is great for the 80/20 rule. Rails will take you the first 80% faster then any other framework. But rails will make it super difficult to get that last 20% and will cost you double the hosting fees(over merb) or more when you have to scale.
I was not happy with Rails for the work I needed to do http://shellshadow.com. I have a very deep background in App Framework development. I wrote the worlds first full-stack app framework in Smalltalk in the early 90s. I also wrote may have been the world's first ORM (also in Smalltalk in 1988). I also wrote similar full stack App Frameworks in Java. I'm out of the framework business now but have a more critical eye than most when it comes to analyzing frameworks.
I have not put together a comprehensive list of why merb is better for me than rails. You will either have to do this for yourself or wait until someone bothers to spend the time to publish compelling and objective analysis. This will come with time, I'm sure.
But if you are happy with Rails, do nothing...keep chugging along. I won't disrespect you for it. Rails just wasn't for me.
I only partially mean it that way ;). What I mean is I do feel qualified to analyze and make this decision myself without requiring others to publish analysis. I did my internal analysis and I chose merb for recoding http://shellshadow.com and 9 months later am still happy with the choice.
I don't mean to sound snob-ish about it though. Everyone has their own unique strengths they bring to working on start-ups. My experience with frameworks is one of mine.
Merb needs to spell out why I should give it a try; what makes it not just a bit nicer than Rails, but way better. If it's just not that much of an improvement, the network effects of having tons of users, plugins, etc... make Rails more attractive.